The present invention relates to a hot runner manifold system for an injection mould for simultaneous injection moulding of several parts or of large parts with a multiplicity of injection nozzles or orifices, which are designed for delivering injection moulding compound substantially simultaneously under high pressure and at high temperatures into specified mould regions, the mould regions being provided with main manifolds that are at least partially separated from one another, which have hot runners and are connected via pre-manifolds, which in turn have hot runners, with one another and with a sprue for the injection moulding compound.
Corresponding hot runner manifold systems and injection moulds have long been known in the state of the art, and corresponding injection moulds are known in particular as moulds for the production of blanks of PET bottles, i.e. of preforms, which consist of PET tubes that are initially relatively thick-walled, and can have very varied shapes, have a relatively small volume and are moulded identically in every respect. The present invention relates in particular to the said hot runner manifold systems for injection moulds for the production of PET blanks, but it can also be applied directly for hot runner manifold systems for other injection moulds, especially if these are intended for the production of a large number of identical parts in one mould or alternatively are designed for the production of very large moulds with a multiplicity of injection nozzles or orifices.
In accordance with the size and large number of parts that can be made with moulds of this kind, the hot runner manifold systems for these moulds are relatively large and heavy. So that, in the case of several products of the same kind, all the mouldings produced have the same quality and finish, it is necessary for the conditions to be exactly the same, i.e. same temperature and same pressure of the injection moulding compound, at all injection nozzles or orifices during injection of the moulding compound. This means, in turn, that there must be the minimum possible loss of temperature and pressure of the moulding compound on its path to the injection orifices. Furthermore, the paths of the moulding compound from a sprue that is connected directly to a storage tank or the like for the moulding compound, as far as the injection orifices or nozzles, should as far as possible be of identical design and as equal as possible in length.
Another problem when using large injection moulds for correspondingly large parts or for a correspondingly large number of individual, smaller parts is that of thermal expansion, which necessarily affects the individual elements of the mould and of the feed systems or pre-manifolds for the molten injection moulding material. Cooling is only possible to a limited extent, because if cooling were excessive there could be local solidification of the injection moulding material, so that runners or injection nozzles could become clogged. Instead, in many cases injection nozzles are even actively heated, so that premature solidification of the moulding material leaving the nozzles is always prevented. The temperature of the injection material can be in the range from 200 to 300° C., so that on account of the large temperature difference relative to room temperature the thermal expansion is appreciable and cannot be prevented completely even by using materials with low thermal expansion. This is one of the reasons why these large moulds are made in several parts, without a rigid connection between the individual mould parts and the corresponding, associated main manifolds, instead they are only fixed at one or a few points and with a clearance between them, so that the thermal expansion of one main manifold does not affect the position of another, adjacent main manifold.
For the same reason, the individual main manifolds and the feeder systems, i.e. the previously mentioned pre-manifolds, have until now been arranged in different planes. Because, on the one hand, the individual main manifolds are supplied with the injection moulding material from a central sprue and to the extent that they must be connected to each other indirectly via pre-manifolds, but rigid connection in the same plane would lead to correspondingly large relative displacements of main manifolds located at a distance from one another, so until now the individual main manifolds have been arranged relative to one another in a first plane and with a clearance from one another without rigid connection, and a pre-manifold was arranged in a plane above or below, and this pre-manifold was for its part connected to a sprue arranged in a further plane above or below. Thermal expansion of the moulds and of the pre-manifolds then only leads to comparatively small mutual effects on one another, because the corresponding parts, arranged in different planes, only have relatively small dimensions in the direction perpendicular to the planes in which they are arranged, so that the absolute changes in thickness still remain within a manageable range even with quite large thermal loads. Usually, in the state of the art, pre-manifolds and the underlying mould parts are bolted together, with bolts passing through slots, and in the cold state the pre-manifolds and moulds are held together by the bolts with little clearance and the mould parts and the pre-manifolds can still be displaced in the direction perpendicular to the bolt extension when there is corresponding heating, because the bolts, as already mentioned, are housed in corresponding slots, which permit such a displacement transversely to the bolts. The clearance between the mating surfaces of the pre-manifolds and of the corresponding main manifolds can be kept small enough so that even in a still not completely hot state of the mould, the relatively highly viscous injection moulding material cannot penetrate into corresponding sealing gaps.
However, this arrangement of main manifolds and pre-manifolds one above the other means that overall the mould becomes relatively bulky and heavy, especially as the mould must generally be fixed in a corresponding machine and must be provided with corresponding connections, and the corresponding holders or seatings and connections for the mould must be adapted to its shape. On account of the relatively large mass of the main manifolds and of the pre-manifolds including all associated components, corresponding heating power is also required, for heating the whole system to a required temperature and maintaining it at this temperature. The corresponding injection moulding machines therefore become relatively large and heavy overall, require corresponding machine foundations and, especially when these main manifolds and pre-manifolds must also still be movable, they require correspondingly large and heavy machine drives.